In reply to Gaupa:
Tom (Southern Man) is perhaps more up to date on these things than I am but I would not be quite so forgiving.
First of all check the T&Cs of your submission. It may be that by sending them the pic you have (perhaps unwittingly) given them a non-exclusive copyright (ie they can use it whenever or wherever they want without paying you). This is a pretty sneaky clause that is in a lot of photo comps too. If that's the case you are stuffed so chalk it up to experience and forget about it.
If that is not in the T&Cs then it is a straight breach of copyright. I don't accept the excuse that they have no budget to buy pix. If they can't afford to pay they should not use it. I don't have a budget to buy a plasma tv but that doesn't mean I can walk into Curry's and help myself to one.
Ring the paper(s) concerned, ask for the picture desk, tell them what has happened and ask for a payment and correction of the credit. They may try to tell you they got it from a bona fide source, some even try to tell you they got it off the internet so it's free. Neither of these excuses the breach of copyright and has no force in law. If they have paid someone else that's their problem not yours. They have used your pic and they must pay.
If that does not work write recorded delivery to the editor(s) of the paper(s) concerned pointing out that the pic has been used without permission and enclose an invoice (say £50?) payable within 30 days and saying interest will be charged thereafter. Details of how to do this are on www.gov.uk/late-commercial-payments-interest-debt recovery
The only reason not to do this would be to preserve a relationship with the paper(s) with a view to getting stuff published in the future but do you really want a relationship with someone who nicks your pix, doesn't pay you, credits them wrongly and then ignores your complaint?